IPA: /ˈtoʊn.ləs/
KK: /ˈtoʊn.ləs/
Not having a tone or sound; lacking energy or expression.
The speaker's toneless voice made it hard to stay engaged during the presentation.
Toneless → It is formed from "tone" (from Old English *tōn*, meaning sound or musical pitch) and "-less" (meaning without). The word "toneless" means without tone or lacking in musical pitch.
Think of being 'without' ('-less') a 'sound' or 'musical pitch' ('tone') — that's what toneless means.